1. Field of the Invention
This invention relates to a semiconductor laser device attaining laser oscillation at a low threshold, said laser device containing a quantum well as an active layer formed by molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), etc., and having an oscillation wavelength in the visible region of 700 nm or less.
2. Description of the Prior Art
With the development of optical information processors such as optical discs, and laser printers utilizing semiconductor laser devices as a signal light source, semiconductor laser devices are required to produce laser light having a short wavelength. Ga.sub.1-x Al.sub.x As semiconductor laser devices have been developed for this purpose. Especially, Ga.sub.1-x Al.sub.x As semiconductor laser devices attaining laser oscillation in the 780 nm region work at room temperature for 10.sup.6 hours or more, and are widely used as light sources for compact discs.
However, the Ga.sub.1-x Al.sub.x As semiconductors crystal tends to transform from a direct transition crystal into an indirect transition crystal as the AlAs mole fraction x in the Ga.sub.1-x Al.sub.x As crystal is increased. Thus, in conventional double-heterostructure semiconductor laser devices in which a Ga.sub.1-x Al.sub.x As crystal is used for the active layer, when the AlAs mole fraction x is 0.2 or more (i.e., x.gtoreq.0.2) and the oscillation wavelength is 750 nm or less, the internal efficiency of the devices decreases, causing an increase in the threshold current level, as disclosed by T. Hayakawa et al., Journal of Applied Physics vol. 54, p. 2209 (1983).
For this reason, the oscillation wavelength, at which conventional double-heterostructure semiconductor laser devices continuously oscillate laser light at room temperature, is 683 nm at their shortest (S. Yamamoto et al., Appl. Phys. Lett. vol. 41, p. 796 (1982)).
Moreover, conventional GRIN-SCH (graded-index separate confinement heterostructure) lasers having a GaAs quantum well with a thickness of 200 .ANG. or less attain laser oscillation with a threshold current density of as low as 200 A/cm.sup.2 or less at a wavelength in the infrared zone of around 800 nm (T. Fujii et al., Extended Abstracts of the 16th Conference on Solid State Devices and Materials p. 145 (1984)).
FIG. 4 shows the distribution of the AlAs mole fraction x a Ga.sub.1-x Al.sub.x As mixed crystal of the GRIN-SCH lasers, indicating that, for example, when the AlAs mole fraction x in the Ga.sub.1-x Al.sub.x As mixed crystal for the quantum well is zero (i.e., the GaAs crystal is used for the quantum well), and when the AlAs mole fraction y in the Ga.sub.1-y Al.sub.y As barrier layer is 0.2 (i.e., the Ga.sub.0.8 Al.sub.0.2 As crystal is used for the barrier layer), and moreover when the AlAs mole fraction z in the Ga.sub.1-z Al.sub.z As cladding layer is 0.5 (i.e., the Ga.sub.0.5 Al.sub.0.5 As crystal is used for the cladding layer), then the quantum well can be provided with an energy barrier which is sufficiently high corresponding to the difference between y and x (i.e., y-x=0.2), and leakage of carrier from the GRIN region to the cladding layer adjacent to the GRIN region can be prevented corresponding to the difference between z and y (i.e., z-y=0.3), and moreover the AlAs mole fraction gradient in the GRIN region having various AlAs mole fractions therein can be set at a high level to thereby effectively lead carrier into the quantum well and effectively converge light into the quantum well.
Semiconductor laser devices oscillating visible light having an oscillation wavelength of 700 nm or less can be produced, using such a conventional GRIN-SCH structure, when, for example, x=0.3, y=0.5 and z=0.8. However, the AlGaAs crystal is converted from a direct transition type into an indirect transition type when the AlAs mole fraction therein becomes 0.45 or more, and an increasing rate of its energy gap decreases with an increase in the AlAs mole fraction therein, compared with the case in which the AlAs mole fraction is 0.45 or less, which causes difficulty in creating a great energy gap. For example, when y=0.2 and z=0.5, the difference in energy gap between the cladding layer and the barrier is 325 meV, while when y=0.5 and z=0.8, the difference in energy gap therebetween is as small as 93 meV so that it is difficult to effectively lead carrier into the quantum well and leakage of carrier to the cladding layer cannot be prevented, resulting in an increase in the threshold current.